A process for isolating pollutants in ground layers is known from EP-B-121 808, in which a gel-forming mixture based on alkali metal silicate and containing at least one alkyltrialkoxysilane is injected into the ground.
EP-A-139 838 discloses pollutant-resistant compositions which, in addition to swellable clay minerals, hydraulic binders, fillers and water, contain an additional amount of alkali metal aluminates and of trialkoxysilanes of the formula R--Si(OR').sub.3, or their hydrolysis products, where R is an aliphatic radical having from 2 to 6 carbon atoms and R' are alkyl radicals which may be identical or different and which have from 1 to 4 carbon atoms.